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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-2991) Index split deadlock
Date Fri, 19 Dec 2008 18:55:44 GMT

     [ https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Mike Matrigali updated DERBY-2991:
----------------------------------


that's great news on initial performance comparisons.  

I am not sure if the following will do better, but it is what I thought of.  It seems like
the worst case would be cases where we move the scan a lot without having latch on page. 
ie. case where we get one row at a time from the page.  So I would suggest using the bulk
fetch property to only get 1 row at a time:
CALL SYSCS_UTIL.SYSCS_SET_DATABASE_PROPERTY('derby.language.bulkFetchDefault','1');

1000 rows does not seem like enough data to do a test.  One of the problems with the new approach,
especially with something like decimal is that there is going to be extra object allocation
per row.  So you want enough rows to see what effect it might have on GC.  I usually tried
to make sure performance test took at least a second, and maybe 60 seconds
for at least a one time run.  

Is a read only test  affected by pages going out of cache?  From your description I don't
think so unless you get very strange cache behavior in that the scan current page goes out
of cache during the scan of that page.  But might be worth showing that a both approaches
perform same on a scan of a
table that is bigger than the cache when the scan is executed multiple times in a row.


Should there be something that exercises the worst case where  the page is updated?  Again
for this having a bigger dataset wil cause more overhead for the 
search.   For this maybe something like a 2 part key with first part being as the current
test
and the second part being an int and the test runs a cursor through the table and after reading
each index row then updates the second part of the key by minus one, likely leaving the key
on the same page - but not putting the scan into an infinite loop.   Make sure the cursor
choice is one which goes to the table every time, not one that caches the
values in the client somehow.   

> Index split deadlock
> --------------------
>
>                 Key: DERBY-2991
>                 URL: https://issues.apache.org/jira/browse/DERBY-2991
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions: 10.2.2.0, 10.3.1.4
>         Environment: Windows XP, Java 6
>            Reporter: Bogdan Calmac
>            Assignee: Knut Anders Hatlen
>         Attachments: d2991-preview-1a.diff, d2991-preview-1a.stat, d2991-preview-1b.diff,
d2991-preview-1b.stat, d2991-preview-1c.diff, d2991-preview-1c.stat, d2991-preview-1d.diff,
d2991-preview-1d.stat, derby.log, InsertSelectDeadlock.java, perftest.diff, Repro2991.java,
stacktraces_during_deadlock.txt
>
>
> After doing dome research on the mailing list, it appears that the index split deadlock
is a known behaviour, so I will start by describing the theoretical problem first and then
follow with the details of my test case.
> If you have concurrent select and insert transactions on the same table, the observed
locking behaviour is as follows:
>  - the select transaction acquires an S lock on the root block of the index and then
waits for an S lock on some uncommitted row of the insert transaction
>  - the insert transaction acquires X locks on the inserted records and if it needs to
do an index split creates a sub-transaction that tries to acquire an X lock on the root block
of the index
> In summary: INDEX LOCK followed by ROW LOCK + ROW LOCK followed by INDEX LOCK = deadlock
> In the case of my project this is an important issue (lack of concurrency after being
forced to use table level locking) and I would like to contribute to the project and fix this
issue (if possible). I was wondering if someone that knows the code can give me a few pointers
on the implications of this issue:
>  - Is this a limitation of the top-down algorithm used?
>  - Would fixing it require to use a bottom up algorithm for better concurrency (which
is certainly non trivial)?
>  - Trying to break the circular locking above, I would first question why does the select
transaction need to acquire (and hold) a lock on the root block of the index. Would it be
possible to ensure the consistency of the select without locking the index?
> -----
> The attached test (InsertSelectDeadlock.java) tries to simulate a typical data collection
application, it consists of: 
>  - an insert thread that inserts records in batch 
>  - a select thread that 'processes' the records inserted by the other thread: 'select
* from table where id > ?' 
> The derby log provides detail about the deadlock trace and stacktraces_during_deadlock.txt
shows that the inser thread is doing an index split.
> The test was run on 10.2.2.0 and 10.3.1.4 with identical behaviour.
> Thanks,
> Bogdan Calmac.

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